Not much more to add here. I guess I could measure it by putting different load resistors on and seeing how much the output dropped, and then figuring out the output resistance (assuming a simple model with a perfect voltage generator and a series resistor, ignoring frequency-related effects). There must be some way to calculate it too though, right? Does anyone have a ready-made formula for this? The output stage is a SE 2A3, cathode biased (and bypassed), through an OPT.

So the output impedance will get divided by the square of the turns ratio, just like the load does. My OPT is 2500:8, which is a ratio of 312.5, so my 2A3's output impedance will also be divided by 312.5. Is that correct?

Then, how do I calculate the output impedance from the operating point? Let's say I'm at the 'classic' 250V/60mA operating point (I'm not, but I haven't measured it since my latest changes, beyond making sure none of the voltages were too high) - how do I calculate the output impedance from that? Is it just the slope of the plate curve at the operating point?

Originally posted by Saurav Not much more to add here. I guess I could measure it by putting different load resistors on and seeing how much the output dropped, and then figuring out the output resistance (assuming a simple model with a perfect voltage generator and a series resistor, ignoring frequency-related effects). There must be some way to calculate it too though, right? Does anyone have a ready-made formula for this? The output stage is a SE 2A3, cathode biased (and bypassed), through an OPT.

Calculating it is useful in the design phase. Once you've got an amp on hand, it's easier and better to actually measure it. Your inclination is correct, just switch from an 8 ohm to a 4 ohm load resistor (on the same xfrmer secondary tap) and see the voltage drop.

The frequency variation of source Z is an interesting and useful thing to know, too, and is trivial to get. The simpler formulas won't be accurate for that.

Yeah. Zo is plate resistance in parallel with any other resistance (for instance if you used a resistance-coupled stage, but obviously that doesn't apply here).
Typical Rp for 2A3 is what, 800 ohms? Zo should be around 3 ohms then (don't forget to add in the resistance of the OPT).

Take a signal generator and add a series resistor to its output so that Z out of generator plus external resistance is 1k ohm. Set the voltage to 1 volts rms ( say 1 KHz) . You now get a current source of 1mA.
Connect the free end to your 8 ohm load which is connected to the OPT. Measure the voltage across the 8 ohm load . It will be in mV. So if you get 0.3mV the output impedance with 8 ohms parallel with it is 0.3mV/1mA = 0.3 ohms.
It is not absolutely correct but pretty close. The actual output impedance will be = (8x0.3)/(8-0.3) = 0.3117 ohm.
You should try this at other frequencies also.
Note : Do not connect any input signal to the amp ! It is best to short the input / plug in a 600 ohm load / turn the volume down to zero etc. but " no input signal".
The signal generator and the mvMeter can be obtained free on the web as software used in conjunction with your sound card.
Try www.dazyweblabs.com and www.rightmark.com amongst others.
Cheers.